There have been proposed a wide variety of quick connect pipe couplings wherein a socket is provided with locking balls, and a plug has a locking recess. Upon insertion of the plug into the socket, the locking balls are received in the locking recess to prevent disconnection of the plug from the socket.
One such quick connect pipe coupling includes a socket provided with a plurality of locking elements and formed with a plug receiving bore. A retainer is slidably movable on the inner wall of the bore to prevent the entry of the locking elements into the bore. When a plug is inserted a given distance into the bore, the retainer is axially moved away from the locking elements to allow the locking balls to be received in a locking recess formed on the plug. A sleeve is movably disposed about the socket. When the locking elements are received in the locking recess, the sleeve is moved over the locking elements to lock the locking elements against removal from the locking recess and thus, prevent disconnection of the plug from the socket (see, for example Reference 1).
Another quick connect pipe coupling teaches the use of an annular member disposed radially outwardly, rather than inwardly, of a plurality of locking elements. The annular member is axially movable on a socket and has a ramp. When a plug is inserted into the socket, the coupling end of the plug radially outwardly urges the locking elements into engagement with the ramp of the annular member. As a result, the annular member is axially moved to allow radial outward movement of the locking elements. The locking elements are thereafter received in a locking recess formed on the plug so as to provide a connection between the plug and the socket (see, for example, Reference 2).
In such a quick connect pipe coupling, the sleeve and the socket may be provided with a slot and a pin, respectively so that the sleeve is free to move in an axial direction only when the pin and the slot are axially aligned with one another. This axial movement allows removal of a plug from the socket (see, for example, References 3 and 4).    Reference 1: Japanese utility model publication No. 57-49030    Reference 2: Japanese utility model publication No. 47-42013    Reference 3: Japanese utility model application publication No. 51-69119    Reference 4: Japanese utility model publication No. 3-42305
In the pipe coupling shown in Reference 2, the annular member is returned to its initial position under the action of a spring after the locking elements are received in the locking recess. With the annular member in its initial position, the ramp of the annular member is engaged with the locking elements to complete connection between the socket and the plug. If an undue pulling force is exerted on the plug, the inclined side wall of the locking recess urges the locking elements in a radially outward direction. This results in axial displacement of the annular member in the same manner as when the plug is inserted into the socket. Under the circumstances, the plug can undesirably be pulled out of the socket.
It is, therefore, an object of the present invention to provide a pipe coupling and a socket therefore, which can securely lock a plug against removal from a socket if a pulling force is exerted on the plug.